.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
.\"
.\" Updated with additions from Mitchum DSouza <m.dsouza@mrc-apu.cam.ac.uk>
.\" Portions Copyright 1993 Mitchum DSouza <m.dsouza@mrc-apu.cam.ac.uk>
.\"
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.\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.TH GETHOSTID 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
gethostid, sethostid \- get or set the unique identifier of the current host
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.B long gethostid(void);
.BI "int sethostid(long " hostid );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR gethostid ():
.nf
    Since glibc 2.20:
        _DEFAULT_SOURCE || _XOPEN_SOURCE >= 500
.\"        || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
    Up to and including glibc 2.19:
        _BSD_SOURCE || _XOPEN_SOURCE >= 500
.\"        || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
.fi
.PP
.BR sethostid ():
.nf
    Since glibc 2.21:
.\"             commit 266865c0e7b79d4196e2cc393693463f03c90bd8
        _DEFAULT_SOURCE
    In glibc 2.19 and 2.20:
        _DEFAULT_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
    Up to and including glibc 2.19:
        _BSD_SOURCE || (_XOPEN_SOURCE && _XOPEN_SOURCE < 500)
.fi
.SH DESCRIPTION
.BR gethostid ()
and
.BR sethostid ()
respectively get or set a unique 32-bit identifier for the current machine.
The 32-bit identifier was intended to be unique among all UNIX systems in
existence.
This normally resembles the Internet address for the local
machine, as returned by
.BR gethostbyname (3),
and thus usually never needs to be set.
.PP
The
.BR sethostid ()
call is restricted to the superuser.
.SH RETURN VALUE
.BR gethostid ()
returns the 32-bit identifier for the current host as set by
.BR sethostid ().
.PP
On success,
.BR sethostid ()
returns 0; on error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.BR sethostid ()
can fail with the following errors:
.TP
.B EACCES
The caller did not have permission to write to the file used
to store the host ID.
.TP
.B EPERM
The calling process's effective user or group ID is not the same
as its corresponding real ID.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.BR gethostid ()
T}	Thread safety	T{
MT-Safe hostid env locale
T}
T{
.BR sethostid ()
T}	Thread safety	T{
MT-Unsafe const:hostid
T}
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
4.2BSD; these functions were dropped in 4.4BSD.
SVr4 includes
.BR gethostid ()
but not
.BR sethostid ().
.PP
POSIX.1-2001 and POSIX.1-2008 specify
.BR gethostid ()
but not
.BR sethostid ().
.SH NOTES
In the glibc implementation, the
.I hostid
is stored in the file
.IR /etc/hostid .
(In glibc versions before 2.2, the file
.I /var/adm/hostid
was used.)
.\" libc5 used /etc/hostid; libc4 didn't have these functions
.PP
In the glibc implementation, if
.BR gethostid ()
cannot open the file containing the host ID,
then it obtains the hostname using
.BR gethostname (2),
passes that hostname to
.BR gethostbyname_r (3)
in order to obtain the host's IPv4 address,
and returns a value obtained by bit-twiddling the IPv4 address.
(This value may not be unique.)
.SH BUGS
It is impossible to ensure that the identifier is globally unique.
.SH SEE ALSO
.BR hostid (1),
.BR gethostbyname (3)
